An inverter is an electric device, with which it is possible to supply a load with a voltage that has a variable frequency. Inverters are typically used in connection with frequency converters. FIG. 1 shows an example of a frequency converter connection. The frequency converter is typically composed of two converters, a rectifier 20 and an inverter 40, between which is provided a direct voltage (Udc) or direct current intermediate circuit 30. The rectifier 20 and the inverter 40 may be located physically separately, and one rectifier may supply a plurality of inverters via a common intermediate circuit 30. An example of a rectifier 20 is a diode bridge, which obtains its supply from an alternating current source U, V, W, which is for instance a 50- or 60-Hz alternating current network (grid), and an example of an inverter 40 is an inverter bridge implemented by means of transistors (e.g. IGBT, Insulated-gate Bipolar Transistor) or other semiconductor switches. The inverter 40 is typically used to adjust the power transferred from the intermediate circuit 30 to a motor 50 or other similar load. Accordingly, as illustrated in the figure, the supply a, b, c from the output of the inverter 40 to the motor 50 is typically a three-phase alternating current connection with three phases a, b and c.
Inverters and, more generally, electric drives comprising inverters typically involve various safety functions which aim to control and ensure the safety of the drive in various operation conditions. As control methods have developed, electric drives without movement sensors have become more common in applications in which traditional solutions have required a feedback coupling, such as a tachometer, from the motor's shaft or from mechanics connected thereto. In this kind of sensorless applications, a possible fault situation of the inverter control system may lead to an overspeed of the motor and, as a result, to severe personal injury and/or considerable damage to property, e.g. in the case of lifts and cranes. Therefore, such sensorless solutions typically employ a separate safety circuit or safety supervision system, which is independent of the electric drive and which supervises or verifies the rotation speed or movement of the motor or mechanics connected thereto independently of the inverter by utilizing alternative speed determination. For example, if the determined alternative rotation speed value exceeds a predetermined threshold value, the safety circuit may activate a safety function, which typically comprises activation of an emergency braking of the drive. In addition or alternatively, the safety circuit may use both the alternative rotation speed value and the rotation speed value determined by the inverter for implementing various safety functions. For example, these two values could be compared to one another, and if they differ, certain safety functions might be activated.
A separate speed measurement adds to the cost of the safety function, however, and therefore some solutions are based on monitoring the voltage and current supplied to the motor by the inverter. The safety circuit thus uses the same signals as the inverter controlling the motor and calculates a signal proportional to the motor's speed. When this signal value exceeds a predetermined threshold value, the safety function is activated. The calculation of the signal proportional to the speed is preferably not dependent on motor parameters and it should substantially differ from the speed estimation algorithm used in the inverter in order that the safety circuit would be able to control the operation of the inverter. One solution is to calculate the signal proportional to the motor's speed simply on the basis of the fundamental wave frequency of the voltage or current vector, i.e. the supply frequency. In the case of a synchronous machine, the supply frequency corresponds directly to the speed of the machine. In the case of an asynchronous machine, there is a deviation corresponding to the slip frequency. This deviation, however, is considered negligible to the correct operation of the safety function in such solutions. An example of a solution utilizing the supply frequency of the machine for the estimation of the rotation speed is disclosed in U.S. Pat. No. 6,745,083.
Speed estimation solutions based on the monitoring of the frequency of the fundamental wave of the supply voltage or current have two problems. In the case of an asynchronous machine, such solutions are incapable of detecting a fault situation of the control of the frequency converter in which the actual speed of the machine has increased to a high value while the control erroneously assumes that the speed is close to zero and the frequency converter is supplying low-frequency voltage to the machine. In this situation the speed estimation based on the monitoring of the frequency of the fundamental wave of the supply voltage cannot detect that the speed of the machine is high, because it can only see the frequency supplied by the frequency converter to the machine. In reality, the operating point is on the wrong side of the breakover point and the speed increases in an uncontrollable manner because the accelerating torque caused by the load (e.g. in crane applications) is greater than the counter-torque produced by the machine. In the case of a synchronous machine, the stator current does not necessarily have a magnetization component and, therefore the current vector length is almost zero during zero moment and it is impossible to detect the fundamental wave on the basis thereof. At zero speed also the voltage vector length is zero and thus the voltage vector does not indicate anything about the speed either.